THE INHIBITION OF NITRIC OXIDE‐MEDIATED RELAXATIONS IN RAT AORTA AND ANOCOCCYGEUS MUSCLE BY DIPHENYLENE IODONIUM

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Abstract
1. The effects of diphenylene iodonium (DPI), an inhibitor of reduced nicotinamide adenine dinucleotide phosphate-dependent oxidases (which generate superoxide anions), were studied on nitric oxide (NO)-mediated responses in isolated preparations of the rat aorta and anococcygeus muscle. 2. In aortic rings, the endothelium-dependent relaxant action of acetylcholine was reduced by DPI (0.3–10 μmol/L) in a concentration-dependent manner and abolished by the NO synthase (NOS) inhibitor L-nitro-NG-arginine methylester (l-NAME; 100 μmol/L). Relaxations induced by sodium nitroprusside (SNP) or NO were not affected by DPI or l-NAME. 3. In anococcygeus muscles, DPI (0.3–10 μmol/ L) as well as l-NAME (5–100 μmol/L) produced concentration-dependent reductions of relaxations produced by nitrergic nerve stimulation. Relaxations induced by NO and SNP were not affected by either DPI or l-NAME. l-Arginine (1 mmol/L) prevented the reduction of nitrergic relaxations by l-NAME but not by DPI. 4. Contractions of anococcygeus muscles elicited by exogenous noradrenaline (1 μmol/ L) were not affected or were inhibited by DPI (0.3–10 μmol/L), but the contractions elicited by noradrenergic nerve stimulation were significantly enhanced by DPI and l-NAME. When noradrenergic contractions had already been maximally enhanced by l-NAME (100 μmol/L), DPI produced no further enhancement. l-Arginine (1 mmol/L) prevented the enhancement of noradrenergic contractions by l-NAME but not by DPI. 5. The efflux of radioactivity induced by field stimulation from anococcygeus muscles previously incubated with [3H]-noradrenaline was not affected by either DPI or l-NAME. 6. Superoxide dismutase (SOD, 100 U/mL) had no significant effects on noradrenergic contractions, nitrergic relaxations, relaxations induced by NO or the actions of DPI in the rat anococcygeus muscle. 7. The results suggest that the effects of DPI in reducing the NO-mediated relaxations produced by acetylcholine in rat aortic rings and stimulation of nitrergic nerves in the rat anococcygeus muscle are due to the inhibition of NOS in these tissues. The effects of DPI were not sensitive to l-arginine, and thus the mechanism of inhibition of NOS differs from that of l-NAME.